PRINCIPLES OF ANIMAL BIOLOGY 59 



Characteristics. 



2a. Vertebrae biconcave. 



26. Quadrate bone immovable. 



2c. Pineal eye fairly well developed. Examine the dorsal side of the 

 head of Sphenodon; note a whitish spot some distance back of the eyes. 



2d. Anus a transverse slit. 



3. Order CROCODILINI. Crocodiles and alligators. 



Characteristics. 



3a. Vertebrae usually concave in front. Material for demonstra- 

 tion will probably not be available. 



36. Fore limbs bear five digits, hind limbs four. Verify in specimens. 



3c. Anal opening a longitudinal slit. Compare with a Rhyncho- 

 cephalian in this respect. 



3d. Quadrate immovable. See alligator skull. 



4. Order SQUAMATA. Snakes, lizards, and chameleons. 



Characteristics. 



4a. Vertebrae usually concave in front. Verify on specimens. 



46. Quadrate freely movable. See skull of snake; also of the "blind 

 worm" Anguis. Advantage of this feature? What is the food of 

 snakes? 



4c. Anus a transverse slit. See Thamnophis, Bascanion, and Sis- 

 trurus or any other snake. 



D. SUBDIVISION OF THE ORDERS 



Orders are divided into families, on the basis of characters less funda- 

 mental than those which furnish the basis for the division of classes 

 into orders. To illustrate the features that distinguish families, a few 

 families of turtles 1 may be used. All the families listed below belong to 

 one order, the Testudinata, and there are several families of this 

 order that are not mentioned. 



Some of the Families of Testudinata 



The characters exhibited by turtles are as follows: 

 1. Neck retractile in vertical plane, or (2) bending laterally. 

 3. Cervical vertebrae without (or with only small) transverse processes, 

 or (4) with strong transverse processes. 



l To THE TEACHER. It is not necessary that all of the families of turtles listed here 

 be used in this exercise. It is suggested that the families represented in the region 

 where the work is being done be used, and that if the teacher is more familiar with 

 other groups than with the turtles other keys be substituted. The main requirement 

 is that tjie characters used be the true family characters and not superficial ones which 

 merely happen to differentiate the families of a particular area. 



